Darrieus type vertical axis wind and water turbines are well known (see, for example, US 2011/0006526). Such turbines have a number of known disadvantages which include low efficiency, a lack of starting torque and excessive vibration. In order to overcome some of these disadvantages, a number of turbine arrangements have been proposed which have mechanisms for adjusting the pitch of the blades of the turbine. The Darrieus turbines have not been as efficient or effective as expected. This is thought to be due to the physical stresses and limitations imposed by a practical design and real wind conditions (which are unlikely to be consistent). Furthermore, it is difficult to make a Darrieus turbine self-starting.
A number of arrangements have been proposed for improving the performance of Darrieus or vertical axis wind turbines.
The angle of attack is the angle between the direction of an oncoming fluid and a reference line on an aerofoil such as a turbine blade. The pitch angle or pitch of an aerofoil such as a wind turbine blade is the angle the aerofoil takes relative to a fixed reference such as the base of the wind turbine, or the horizon. For a wind turbine in wind whose direction is not changing, pitch is a measure of angle of attack. The pitch may also be defined as an angular rotation applied to the root of a blade. It is recognised that the pitch of an aerofoil has an impact on the torque exerted on the aerofoil by an oncoming fluid (e.g. wind on a wind turbine) and hence on the output from a turbine including aerofoil blades. Various arrangements for controlling the pitch of the blades of a vertical axis wind turbine are known. Arrangements such as those disclosed in US 2011/0006526 A1, WO 2011/130797 A1, WO 2011/144830 A1, FR 2 924 182 A1, U.S. Pat. No. 3,382,931 A1 and WO 201/0305569 A1 aim to control the pitch of the blades of a vertical axis wind turbine using a cam arrangement whereby a centrally located cam is provided and a cam follower coupled to the blades follows the cam and thereby pivots the blade as it moves around the turbine's axis of rotation.
WO 2011/130797 discloses a wind turbine with pitch control means in the form of a ring or guide located around the shaft and coupled via links or lines to the trailing end of each blade to control the pitch of each blade during rotation of the turbine rotor. The ring or guide is mounted so that it can itself move as the turbine rotates. The mechanism to allow the ring or guide to move creates a passive control system such that the position of the blades is partially determined by the strength and direction of the wind and thereby at least partially self-adjusting. This arrangement is complicated to make and prone to failure. Passive control systems such as that of WO 2011/130797 also cannot provide control of the camber of the blade.
FR 2 924 182 discloses a wind turbine with a cam arrangement to control pitch. A central irregular cam track is provided around the turbine rotor's axis of rotation and cam followers are connected by arms or links to the blades. The arrangement of a single central irregular cam track with a series of links on cam followers results in a complicated arrangement which vibrates and shakes, and is very difficult to adjust for different wind conditions. The control rods or links connecting the central cam followers to the respective blades are shown as being parallel to the respective support arms. However, those rods or links would need, in use, to pull the blades to angles which are not parallel to the arm. This is likely to lead to high frictional forces and vibration and shaking.
U.S. Pat. No. 3,382,931, US 2011/0006526, WO 2011/144830 and US 2011/0305569 all disclose arrangements similar to the original arrangement proposed by Darrieus in which pitch control is achieved by means of providing links or rods connected to a central pitch control unit such that they rotate about a point eccentrically located relative to the turbine rotor's axis of rotation. Such arrangements also suffer from excessive vibration and hence losses.
The inventor of the subject application has appreciated that the arrangements with rigid rods or links connected to a shared central hub or pivot result in strains and vibrations resulting from the fact that opposite sides of the turbine are subject to very different wind conditions and opposing links or rods therefore exert different forces and give rise to imbalances. Pitch control devices for vertical axis wind turbines have had no commercial success. There have been many designs and, of the available test data, none have been promising. There have been problems with vibration, friction and poor performance, which the existing and known arrangements have not sufficiently resolved.
Although variable pitch appears to be an answer to the large variations in the angle of attack of a vertical axis wind turbine during each revolution of the turbine (and the resultant significant fluctuations in dynamic loading that cause fatigue and reduce turbine performance) no successful design has been found. Known vertical axis turbines are therefore not as powerful as they might be and there are fatigue implications in the current vertical axis designs that do not have pitch control.
U.S. Pat. No. 3,382,931, US 2011/0006526, FR 2 924 182, WO 2011/130797 and US 2011/0305569 disclose wind turbines in which blade pitch angle of a number of blades is controlled by rods connecting each blade to a central cam track or a single central bearing which rotates about an axis offset from the turbine blade's axis of rotation. As discussed above, the inventor of the subject application is the first to appreciate that this arrangement of rods extending out to a number of blades from a single central track or bearing results in poor performance. The reasons for the poor performance are not entirely clear but it is believed that it is because, in practice, blades and rods are not perfectly rigid there are inevitably slight changes in the distance between blades as the blades rotate because of differences in the forces at different points in their rotation. These differences lead to vibrations. The faster the rotation and/or wind or fluid speed, the greater these effects will be as the differences in the forces between different points in the rotation become bigger.
The camber of an aerofoil such as a turbine blade is effectively the shape or curvature of the aerofoil. The camber of an aerofoil can be defined by a camber line, which is the curve that is halfway between the upper and lower surfaces of the aerofoil. US 2010/0096854 A1 discloses a vertical axis wind turbine which can be actively controlled. Each blade is in two parts and the pitch and camber of each blade is controlled by motors arranged in or near the blades which controllably move the two parts of each blade and thereby change the blade pitch and camber. This arrangement requires complex data processing to control each motor. Furthermore, the motors need to be powerful and are difficult to control accurately as the forces necessary to move the blade portions as they rotate are significant.
US 2010/0054936 and DE 20 2010 002 046 U also disclose arrangements for controlling the camber (and/or the pitch) of individual wind turbine blades using computer controlled actuators located in or near the turbine blades. As discussed above for US 2010/0096854, in use the blades move quickly and are subject to significant forces. Actuators or motors suitable for controlling the blades must therefore be able to provide very high forces so as to change the angle of attack or pitch, and the shape or camber, of the moving blades. The necessary actuators or motors are therefore expensive and prone to failure. Furthermore, locating the actuators at or near the blades makes maintenance or replacement of the actuators difficult, time-consuming and expensive.
WO 2004/079187 discloses a wind turbine with passive camber guidance. This does not allow for accurate control of camber and hence optimisation of the camber properties at different points in the rotation of the respective blade.